There are two basic types of rechargeable alkaline cells. These are nickel cadmium (NiCd) cells and nickel metal hydride (Ni-MH) cells. Both types of cells utilize positive and negative electrodes and the positive electrodes of both types of cells are composed of a nickel hydroxide material. At present, both sintered and pasted positive electrodes are used in NiCd and Ni-MH cells.
Sintered positive electrodes are constructed by applying a slurry, comprising nickel powder, to a substrate. This is followed by high temperature sintering whereby the individual nickel particles weld at their points of contact, resulting in a porous material. The sintered material is then impregnated with active material. Sintered electrodes have the advantage of being able to withstand shock and vibration because the active material sticks very well to the substrate. However, sintered electrodes have the disadvantage of having a comparatively low energy density. In order to increase energy density the current trend has been away from sintered electrodes toward the pasted ones.
Pasted electrodes using a nickel foam substrate exhibit satisfactory energy density as well as ruggedness. Such pasted positive electrodes are useful in both NiCd and Ni-MH rechargeable cells. The nickel foam substrate is manufactured by nickel plating polyurethane foam and then burning off the polyurethane. The resulting nickel foam is then loaded via a slurry with nickel hydroxide, the active ingredient of the positive electrode.
Manual and automated methods of pasting foam are known in the art. Manual methods tend to be slow and tedious. The process begins by combining and mixing the ingredients, adjusting the viscosity of the slurry for maximum loading, pouring the slurry over the foam substrate, and then spreading it over the surface of the foam with a spatula.
Certain properties of the foam affect loading. Variations in such factors as foam pore size, porosity, and depth affect how well the slurry can penetrate through the foam after it is poured onto one side of it. Such factors also determine the viscosity of the slurry that must be used in the loading process, as well as the amount of effort needed to force the slurry into the foam to achieve maximum loading. Very slow seepage may even require that the slurry be applied and pushed in from both sides of the foam, greatly increasing the time and effort needed to complete the loading process.
Besides the inherent slowness of the manual loading process, it has other disadvantages. The time needed to complete the process gives the slurry suspension time to settle. Hence, the viscosity of the slurry may change during the course of the loading process, resulting in electrodes exhibiting nonuniform densities of active material.
Hand loading, like all "by hand" operations is dependent on the individual performing the operation. If time and cost are not a factor, a skilled craftsman can produce positive electrode material of the highest quality.
Automatic loading can speed the process of applying slurry to foam. Several types of automated loading methods are used in practice. The most common are those which rely on mechanically rubbing or forcefully spraying the slurry into the foam.
Spraying methods automatically load the foam by spraying it from either one or both sides. One such method/apparatus is described in U.S. Pat. No. 4,582,098 (to Matsumoto et al, issued Apr. 15, 1986). The major problem with spraying (and especially those processes that rely on nothing more than spraying the slurry onto the foam from a single side) is that it results in a nonuniform application of the active nickel hydroxide ingredient into the foam substrate.
If a slurry, supplied at a certain flow rate, is simply sprayed from a single nozzle onto one of the surfaces of a porous foam substrate material, the liquid and solid components making up the slurry suspension separate because the liquid portion of the slurry penetrates more easily through the pores of the foam. This results in a nonuniform distribution of the nickel hydroxide active material. Slurry containing a higher concentration of solid will deposit near the surface that is closer to the nozzle, while slurry with a lower concentration of solid will penetrate deeper into the interior of the substrate material. Hence, spraying slurry with a single nozzle from just one side is unsatisfactory from the standpoint of a uniform concentration of active material through the entire volume of the foam.
There are other problems associated with automated loading that rely solely upon a spraying process. The filling density of the foam is sensitive to factors directly related to the spraying system used. For example, the filling density is affected by such factors as the amount of air that enters the slurry, the flow rate of the slurry at the nozzle port, and the distance between the foam and the spray nozzle spout.
Spraying the slurry onto both sides of the substrate, as disclosed in U.S. Pat. No. 4,582,098, fails to overcome all of these problems. Variations in filling density introduced into the process by the spray mechanisms are still present in such a two nozzle system. Furthermore, even when spraying from opposite sides some of the slurry that is sprayed at the surface rebounds and never penetrates the porous interior.
Thus, a loading system is needed that will combine the speed and noninvasiveness of an automated spray method with the superiority of loading seen in "by hand" application.